MicroRNAs (miRNAs) are a new class of noncoding RNAs, which are encoded as short inverted repeats in the genomes of invertebrates and vertebrates (Ambros, (2001) Cell 107, 823-826; Moss (2002) Curr. Biol. 12, R138-R140). miRNAs are modulators of target mRNA translation and stability, although most target mRNAs remain to be identified. miRNAs sequence-specifically control translation of target mRNAs by binding to sites of antisense complementarity in 3′ untranslated regions (UTRs) (Ambros, supra; Moss, supra; Lagos-Quintana et al., (2001) Science 294, 853-858; Lau et al., (2001) Science 294, 858-862; Lee et al., (2001) Science 294, 862-864).
Several miRNAs, such as let-7 RNA, miR-1, miR-34, miR-60, and miR-87, are highly conserved between invertebrates and vertebrates, implicating that they may recognize multiple sites and/or multiple targets of presumably conserved function (Lagos-Quintana et al., supra; Lau et al., supra; Lee et al., supra; Pasquinelli et al., (2000) Nature 408:86). The small temporal RNAs (stRNAs) lin-4 and let-7 represent a subclass of miRNAs identified by genetic analysis in Caenorhabditis elegans, which are developmentally regulated and themselves control developmental programs, such as timing of neuronal rewiring, Dauer larva formation, vulva formation, and the terminal differentiation of hypodermal cells.
miRNAs are typically excised from 60- to 70-nucleotide foldback RNA precursor structures, which are sometimes detected at the onset of miRNA precursor expression (Grishok et al., (2001) Cell 106, 23-34; Hutvagner et al. (2001) Science 93, 834-838; Ketting et al., (2001) Genes Dev. 15, 2654-2659) or during expression of very abundant miRNAs (Lagos-Quintana et al., supra; Lau et al., supra; Lee et al., supra). Generally, only one of the strands of the hairpin precursor molecule is excised and accumulates, presumably because it is protected by associated proteins from RNA degradation. These putative proteins may mediate the translational suppression. The miRNA precursor processing reaction requires Dicer RNase III and Argonaute family members (Grishok et al., supra; Hutvagner et al., supra; Ketting et al., supra).
In addition to their impact on gene expression, these small RNAs, often in the range of 21-22 nucleotides, may find utility in areas of therapeutics and drug discovery, e.g. as drug targets or as pharmaceutical agents. Thus, in some circumstances, it may be important to know approximately how much of each miRNA exists in cells. In some cases, it may further be important to compare levels of miRNA in different tissue types or before and after application of a stimulus, e.g. a chemical or physical intervention. Because related siRNAs and miRNAs may be present in low amounts in cells, it is desirable that methods of detection be both sensitive and specific. Moreover, for certain applications, it may be beneficial to identify methods suitable for high throughput screening, e.g. homogeneous methods, multiplexed methods, or those suitable to highly parallel automated manipulation and limited temperature changes.
Although miRNAs play important roles in the regulation of gene expression, effective techniques for the detection and quantitation of miRNA expression are lacking To date, the principal methods used for quantitation of miRNAs are based on gel electrophoresis. The miRNAs are detected either by Northern blotting or by the presence of radioactive RNase-resistant duplexes. Northern blotting and chip hybridization methods have relatively low analytical sensitivity (Krichevsky et al. 2003), so microgram quantities of RNA are needed for analyses; moreover, transfer of small RNAs to filters can introduce problems with reproducibility of quantitation and is not typically amendable to high-throughput. Moreover, detection methods based on RNase resistance require highly radioactive probes. Further, assays based solely on probe hybridization may not provide adequate discrimination between isotypes closely related in sequence. Alternative approaches involve cloning the miRNAs and then sequencing the inserts. While this approach may be suitable for discriminating single-base differences between closely related miRNA species, it is time consuming and laborious.
Like miRNAs, small interfering RNAs (siRNAs) are small RNA molecules involved in cell defense, e.g. against viral RNA, via a response termed RNA interference (RNAi) (Cullen, B. R., Nature Immunology, 3: 597-599 (2002)). One class of siRNAs is produced through the action of the Dicer enzyme and RNA-induced silencing complex (RISC) protein complex as part of the RNAi response to the presence of double stranded RNA in cells (Khvorova, A. et al., Cell 115: 209-216 (2003)). Another class of siRNAs is synthetic and encompasses short duplexes, usually 21-23 nt with characteristic dinucleotide overhangs (Elbashir, S. M. et al., EMBO J. 20: 6877-6888 (2001)) introduced directly into cells via transfection or expression from an introduced vector (Paul, C. P. et al., Nature Biotechnology 20: 505-508 (2002), U.S. Patent Application Publication No. 2003/0148519A1, herein incorporated by reference in its entirety for all purposes). In some cases, siRNAs appear to persist as defined sequences, making them analogous in function and composition to miRNAs (Elbashir, S. M. et al., supra). What is needed are efficient and accurate methods of detecting and quantitating miRNA and siRNA levels.